The present invention relates to a method of sampling color out-of-registration detect patterns formed on a rotating, endless image bearing member in a color image forming apparatus, such as a digital color copying machine or a digital color printer, which forms, for color image formation, a plural number of toner images of different colors on an image transfer member placed on the endless image bearing member or directly on the endless image bearing member.
Color documents are rapidly superseding black and white documents in recent offices. With this trend, image forming machines handling these documents, such as copying machines, facsimile machines, and printers, are also superseded by color image forming machines. Further, high image quality and high operating speed are required for recent document processings in recent office works. To satisfy the requirements, the related machines producing high quality pictures at high speed are proposed and developed. One of such machines is provided with three image forming units for black (K), yellow (Y)., magenta (M), and cyan (C). Color images formed by those units are transported, in a superimposed fashion, on a transfer member or a medium transfer member being transported. This machine is of the so-called tandem type.
A specific example of the color image forming apparatus of the tandem type follows. As shown, the color image forming apparatus includes four image forming units; a black image forming unit 100K for forming a black (K) image, a yellow image forming unit 100Y for forming a yellow (Y) image, a magenta image forming unit 100M for forming a magenta (M) image, and a cyan image forming unit 100C for forming a cyan (C) image. These image forming units 100K, 100Y, 100M, and 100C are horizontally disposed at fixed intervals. An endless transfer belt 102 is disposed under the image forming units 100K, 100Y, 100M, and 100C of four colors, black, yellow, magenta and cyan. The endless transfer belt 102 transports a transport paper 101 through the image transfer positions of those image forming units, while electrostatically attracting it thereonto. The endless transfer belt 102 is made of flexible synthetic resin, such as PET.
The four image forming units 100K, 100Y, 100M and 100C of black, yellow, magenta, and cyan have substantially the same constructions. Those units form toner images of black, yellow, magenta, and cyan in successive manner. The units are each provided with a photoreceptor drum 103. The surface of the photoreceptor drum 103 is uniformly charged by a scorotron 104 for primary charging. Then, the surface of the photoreceptor is exposed to a laser beam 105 for image formation, which repeatedly scans across the surface thereof in accordance with image information. As a result, a latent electrostatic image is formed on the surface of the photoreceptor. The latent electrostatic images thus formed on the photoreceptor drum 103 are developed into visual color images by toner of black, yellow, magenta, and cyan. These visual color images are subjected to a before-transfer charging process by a before-transfer charger 107, and successively transferred onto the transport paper 101 held on the endless transfer belt 102. The transport paper 101 having color toner images of black, yellow, magenta, and cyan transferred thereonto is separated from the endless transfer belt 102, and subjected to a fixing process by a fixing unit, not shown). In this way, a color image is formed.
In the figure, reference numeral 109 designates a photoreceptor cleaner; 110, a photoreceptor discharging lamp; 111, a paper detach corotron; 112, a transfer belt discharging corotron; 113, a transfer belt cleaner; and 114, a pre cleaning coroton.
In the thus constructed color image forming apparatus of the tandem type, a plural number of image forming units are used for forming one color image. Because of this construction, the color forming speed may be increased to some extent. However, the apparatus suffers from such a problem that where the color forming speed is increased, relative positioning of the color images formed by the image forming units, viz., a color registration, is frequently deteriorated and the picture quality of the resultant color image is also deteriorated. In this respect, it is technically difficult to improve both the picture quality and the image forming speed. This problem arises from the fact that a variation of temperature, within the apparatus and/or application of an external force to the apparatus causes the positions and the size of the image forming units per se, and the component parts of the units to displace. The temperature variation and the application of the external force inevitably take place. In ordinary works for the image forming apparatus, such as removal of paper jamming, parts exchange in maintenance, or movement of the color image forming apparatus, an external force is inevitably applied to the apparatus.
Published Unexamined Japanese Patent Application No. Hei. 1-281468 discloses another color image forming apparatus. The apparatus is constructed with a plural number of image forming sections for forming visual images representative of image information on an original document, and additionally other visual images of position marks for position detection, a moving member which moves along image transfer ares where the visual images of original image information formed by the image forming sections and the visual images of the position marks are transferred, and mark detecting means for detecting the position marks transferred on the moving member, the mark detecting means being located downstream of the moving member in the image transfer areas when viewed in the direction of the movement of the moving member. The thus constructed color image forming apparatus controls the image forming sections so as to correct out-of-registration of the transferred images in accordance with detect signals output from the mark detecting means.
An example of the technique for correcting the out-of-registration of the transferred images, when applied to the tandem type color image forming apparatus shown in FIG. 17, is illustrated in FIG. 18. As shown, color out-of-registration detect patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are arrayed at proper spatial intervals in the advancing direction of the endless transfer belt 102 and in the direction orthogonal to the belt advancing direction. A light emitting element 123 illuminates these patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C from the underside of the patterns in the illustrated example. A line photosensing device 122, located above the patterns, samples the patterns by using light transmitted through the transfer belt. The line photosensing device 122 may be a CCD sensor containing a number of photosensing picture elements linearly arrayed. Distances each between the adjacent patterns of those patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are computed. The positions of the image forming units 100K, 100Y, 100M and 100C and the image forming timings are adjusted so that the computed distances are equal to the corresponding reference values. In this way, the picture quality of the resultant picture is improved.
In the color image forming apparatus thus constructed, the color out-of-registration detect patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are formed by the image forming units 100K, 100Y, 100M and 100C. Those patterns are detected by the line photosensing device 122, such as a CCD sensor. The distances each between the adjacent patterns of those patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are computed. The positions of the image forming units 100K, 100Y, 100M and 100C and the image forming timings are adjusted so that the computed distances are equal to the corresponding reference values. In this way, a high picture quality of the resultant picture is realized.
When a CCD sensor 122 as a line photosensor is used for detecting the color out-of-registration, the sensitivities of the photosensing picture elements linearly arrayed are not uniform as shown in FIG. 19. Accordingly, when the positions of the patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are detected by the line photosensing device 122, the detection results suffer from detection errors because of the different sensitivities of the photosensing picture elements.
Unexamined Japanese Patent Publication No. Hei. 6253,151, filed by the applicant of the present Patent Application, proposes a unique technique for removing the detection errors. The technique applies a shading correction to the output signals of the CCD sensor 122 for removing the detection errors when the positions of the patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are detected. In the shading correction, the photosensing picture elements of the CCD sensor 122 are illuminated with light at a fixed light intensity. The signals output from those picture elements are corrected so as to be uniform in value by a computing processing by a CPU, for example.
However, the conventional art has the following problem. In the conventional art, as described above, the shading correction process is applied to the output signals of the picture elements of the CCD sensor 122, in order to prevent the detection error that will be caused when the CCD sensor 122 as a line photosensing device detects the positions of the color out-of-registration detect patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C. Use of the shading correction process improves the position detection accuracy of the out-of-registration detect patterns, but increases a load to the CPU when it operates for computating. As a result, the hardware and the software to cope this increased load to the CPU are large in construction, leading to the increase of cost to manufacture.
In the attempt of realizing the shading correction process without increasing the hardware and the software, time taken for the shading correction process is long, and it is impossible to reduce the sampling period in the sampling of the color out-of-registration detect patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C. For this reason, the sampling period must be set long to some extent. Under this condition, the AC components of the signals representative of the out-of-registration of the image forming positions of the respective colors that are detected on the basis of the results of the sampling of the patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C are limited to those of low frequencies. As a result, the detection error of the DC component which indicate overall out-of-registration of those color image forming positions frequently takes place, as shown in FIG. 20. In the conventional art, the results of the sampling of the color out-of-registration detect patterns 120K, 120Y, 120M and 120C, and 121K, 121Y, 121M and 121C, which greatly contain the DC component detection errors, are used for correcting the DC color out-of-registration components by controlling the image forming timings in the black image forming units 100K, 100Y, 100M, and 100C. This fact leads to deterioration of the shading correction accuracy. On the other hand, where the sampling period of sampling the patterns 120K, 120Y, 120M and 120C is short, the DC component detection error is little caused as shown in FIG. 21.